1. Field of the Invention
The present invention is related to an apparatus for observing tomographic structures using light. Particularly, the present invention is related to a probe for observing tomographic structures within living organisms.
2. Description of the Related Art
The utilization of images during medical examinations is becoming widespread. Consequently, the importance of non invasive, non contact observation techniques is increasing.
Conventionally, non invasive non contact observation and measurement of data within subjects such as living organisms had been performed by X rays. However, utilization of X rays has problems, such as the fact that subjects are irradiated with radiation, and that it is difficult to image the biological functions of the subjects. As a result, ultrasound endoscopic probes are in wide use for observation of tissue within body cavities. However, the spatial resolution of ultrasound endoscopic probes is not high, and it is not possible to obtain information of biological tissue except the shape thereof. Further, media such as water are necessary to utilize ultrasound endoscopic probes, and therefore, the processes involved in observation of subjects are complex.
Recently, various techniques related to OCT (Optical Coherence Tomography), which uses light to visualize information regarding structures within subjects' bodies have been proposed. PCT Japanese Publication No. 2005-533610 and Japanese Unexamined Patent Publication No. 2004-223269 disclose examples of OCT endoscopic probes. These OCT endoscopic probes are inserted into body cavities, then optical scanning is performed to image tomographic images of organ walls and the like.
Specifically, PCT Japanese Publication No. 2005-533610 discloses an OCT endoscopic probe. As illustrated in FIG. 6 attached to the present application, the OCT endoscopic probe includes: a transparent cylindrical sheath 10; a single mode optical fiber 11 which is rotatably drivable; a lens 12 fixed to the tip of the optical fiber 11; and a beam director 13 (return mirror). The optical fiber 11, the lens 12, and the beam director 13 are housed within the sheath 10. Note that in this example, the beam director 13 is provided separate from the lens 12. Light that propagates through the optical fiber 11 and is emitted from the lens 12 in a converged state is reflected by the beam director 13, and output toward the outer peripheral directions of the sheath 10.
Japanese Unexamined Patent Publication No. 2004-223269 discloses another OCT endoscopic probe. This OCT endoscopic probe includes: a single mode optical fiber 20; a lens unit 21; and a transparent cylindrical sheath 22, as illustrated in FIG. 7. The tip of the optical fiber 20 and the lens unit 21 are housed within the sheath 22. In this OCT endoscopic probe, the lens unit 21 is constituted by: a GRIN (Gradient Index) lens 24, for focusing the light 23 output from the tip of the single mode optical fiber 20 onto living tissue; a Faraday rotator 25, for cancelling the effects of polarization due to bends in the single mode optical fiber 20; and a prism 26 for changing the direction that the light travels in. The single mode optical fiber 20 and the lens unit 21 rotate with the longitudinal direction (the longitudinal axis) of the sheath 22 as their rotational axis as indicated by arrow 27 of FIG. 7, to scan light in a cylindrical direction. Thereby, observation of measurement targets is enabled not only in the cross sectional direction but also as discoid shapes.
It is necessary for OCT endoscopic probes for medical and non medical use to have extended work distances (distances between light emitting ends and observation targets) and small spot sizes, in order to search through thin twisted paths such as arteries, veins, and pulmonary airways. It is expected that this demand will become even greater in the future. However, there is a problem that it is difficult for the OCT endoscopic probe disclosed in PCT Japanese Publication No. 2005-533610 to achieve a great work distance and small spot size.
Generally in OCT endoscopic probes, the correlations between reflective intensities along optical paths and optical path lengths are detected, which is the basic principle of OCT. Therefore, in cases that two surfaces that both reflect light strongly near each other, light is multiply reflected by these two reflective surfaces. This multiple reflection causes ghost images of the reflective surfaces at portions that do not actually exist, which significantly deteriorates observation properties. Particularly in cases that ghost images are displayed at the same locations as living tissue, it becomes impossible to distinguish between actual images of living tissue and the ghost images, which is a great problem.
Meanwhile, in the OCT endoscopic probe disclosed in Japanese Unexamined Patent Publication No. 2004-223269 (refer to FIG. 7), a light beam 23 which is output from the single mode fiber 20 is reflected a plurality of times at the boundary surfaces of optical elements such as the Faraday rotator 25, the GRIN lens 24, the prism 26, and the sheath 22, then returns to the single mode optical fiber 20. Therefore, images of living tissue and ghost images are overlapped, and there is a problem that the image quality of the images of living tissue is poor.